The U.N. mediator for Kosovo has recommended independence for the breakaway Serbian province. From U.N. headquarters, correspondent Peter Heinlein reports Serbian officials called the decision "unacceptable".

In a long-awaited report to the U.N. Security Council, Special envoy Martti Ahtisaari says "independence is the onlyviableoption for a politically stable and economically viable Kosovo."

At the same time, Ahtisaari says the region is not yet ready to tackle challenges such as protection of minorities, economic development, and social reconciliation.

He recommends an international civilian and military presence be maintained in the region for an unspecified 'initial period', until Kosovo has the capacity to stand on its own.

Kosovo's 90 percent ethnic-Albanian majority hailed the decision, but Serbian President Boris Tadic Monday called any form of independence for Kosovo "unacceptable". He said Serbia remains ready to 'constructively engage' in further talks on the province's future.

U.N. Secretary-General Ban Ki-moon is in the Middle East, but spokesperson Marie Okabe says he accepts Ahtisaari's concept of 'supervised independence.'

"The Security Council has been presented with a plan which the secretary-general believes contains all of the right elements for fair and sustainable solution to Kosovo's future status," she said.

U.S. Undersecretary of State Nicholas Burns was in Brussels when the report was released. He expressed Washington's full support for Ahtisaari's conclusions.

Burns explained 'supervised independence' as a means to provide Kosovo a 'way forward' toward independence, while ensuring the rights of the region's ethnic-Serb minority.

"The European Union would provide economic assistance for a period of time, NATO troops would continue to provide security and that there would be provisions for security for the Serb population for its churches, for its monasteries and for the people themselves, but that we have to get on and see that Kosovo become an independent state," he said. "And the United States supports that process."

Burns said he thinks the U.N. Security Council could pass a resolution confirming Kosovo's future status either in April or May.

Agreement in the Council is complicated by veto-wielding Russia's insistence that any Kosovo solution be acceptable to both sides. Ahtisaari has said he reached his recommendation for independence only after determining that the two sides were so far apart that a mutually acceptable solution was impossible.

Nevertheless, Burns remains optimistic. He told reporters he expects five to seven weeks of consultations to find the best way forward before the Security Council votes.

Kosovo has been under U.N. administration since 1999, after a three-month NATO bombing campaign drove Serb forces from the former Yugoslavia out of the province, ending a deadly Serb crackdown on ethnic Albanians.

The Ahtisaari plan sets the stage for eventual full independence for the region, including provisions for a constitution, a flag, an army, and guarantees that minority Serbs would be allowed to run their own affairs. The plan also gives Kosovo the right to join international organizations reserved for sovereign states.